Client application doesn't end if connection isn't closed properly

[sburnicki]

Without closing the SqpConnection a client program does not exist, since the SqpConnectionImpl doesn't exits the sendingService.

Make sure that the SqpConnection reference is passed back to a client so that it's garbage collected if the client application finishes. Then we can close the sendingService on GC. Currently this probably doesn't work as the SqpConnection reference is also used internally by other objects, so it's not GC.